Light/medium duty vehicles equipped with hydraulic disc brakes have a significant safety issue related to their ability to stop when the hydraulic or power assist system fails. This problem is presently shared by nearly every make and model numbering in the millions. The problem is that the emergency braking capabilities of these vehicles are not adequate. While the current hydraulic braking systems are relatively safe through redundancy, the possibility for complete failure exists and is not too improbable to overlook.
The most commonly used braking system for light/medium duty vehicles is a hydraulic braking system. This system generates its braking fluid pressure by means of a master cylinder, power assistance provided by the power steering pump that is mounted on and powered by the engine. In the event of an engine failure, the engine mounted hydraulic pump will cease to function. Generally a back up electric powered hydraulic pump is used to supply hydraulic-assist pressure to the service brake. However, a safety issue arises when there is a complete failure of the vehicle's electrical system, in that a failure will disable the engine. Further, the back-up electric hydraulic-assist pump will not receive power and will fail to operate when braking is needed. The drive line mounted parking brake, at best, will hold a vehicle parked. However, it is completely incapable of bringing the vehicle to a safe stop within a reasonable distance. This existing system rests in the released position and requires input energy generally by means of a cable to apply.
A large majority of medium duty vehicles, including school buses employ the above-mentioned system. Current parking brakes on medium duty vehicles are prone to failure and do not provide enough holding power. Furthermore, current parking brake systems consist of a manually operated drum brake that is installed somewhere on the drive shaft. Being mounted on the drive shaft, this system is insufficient due to the properties of spider gears in the differential. An example is one rear wheel on a slick surface or in the event a driveline failure occurs. Another type of braking system available for medium duty vehicles is an air braking system. Air brakes do not have the above-mentioned problem because they fail to a safe mode. That is, failure of an air brake system by design will allow spring force to apply the brakes. However, air brakes are much more expensive than hydraulic systems and fall under heavy federal mandates to include required commercial drivers license and strict, frequent maintenance measures.
Thus there is a need for an emergency brake/parking brake system that is completely independent of the service brake system and can be easily and immediately employed on medium duty vehicles to include but not limited to new production and retrofit.